In order to obtain high image quality, the toner used in an electrophotographic process must have sufficient fluidity. The flow characteristics of the toner are critical to the developing step and the cleaning step. Thus, the toner must be in the form of discrete particles and not agglomerates. A common strategy for controlling and maintaining the fluidity of toner is to add metal oxide particles, such as silica, alumina, and titania, thereto.
Spherical or substantially spherical metal oxide particles are most efficient at improving the fluidity of toner. See, for example, U.S. Pat. Nos. 5,422,214 and 6,479,206. Substantially spherical metal oxide particles act as intervening spacers and reduce the adhesion force between the toner particles by increasing the distance and decreasing the contact area between the toner particles, thereby reducing agglomeration. Preferably, the substantially spherical metal oxide particles are nearly the same size, i.e., diameter, as the toner particles to produce a stable toner composition that does not separate into component toner particles and metal oxide particles.
Additional properties of the metal oxide particles, such as surface area, tribo-charging, and environmental stability, also contribute to the performance of the toner composition. One useful approach for customizing the properties of metal oxide particles for use in toner compositions is to prepare composite metal oxide particles that contain a metal oxide core with a metal oxide a coating. The composite metal oxide particles advantageously combine the preferred properties of the core and the coating.
However, a need still exists for suitable toner compositions and for relatively simple and economical methods of preparing the same. The invention provides such a composition and method. These and other advantages of the invention will be apparent from the description of the invention provided herein.